The transmission of electromagnetic signals from a bore-hole to the earth surface is an effective method of communicating information during various types of drilling operations, such as measuring while drilling (MWD) and/or logging well drilling (LWD). The ability to communicate allows for the monitoring of drilling operations, as well as the inspection and evaluation of surrounding geology. During directional drilling operations, such as boring holes under river beds, subways, unusual earth formations and tapping oil reservoirs, it is particularly important at all times to know precisely the location of the drill bit. A significant effort has been made to develop electrical instruments which are capable of transmitting signals at the drill face or inspection face back to the earth's surface.
A number of systems have been developed which incorporate electromagnetic technology for communicating to the earth surface. For example, in U.S. Pat. No. 5,394,141, described is a system where the lower portion of the drill string is used as an antenna for purposes of transmitting electromagnetic waves carrying information.
Various types of devices which are mounted on the outside of the drill string for monitoring surrounding conditions and/or used in communication are described, for example, in U.S. Pat. No. 4,684,946 to Geoservices and U.S. Pat. No. 5,467,832 to Schlumberger Technology Corporation. The problem with mounting communication devices and sensing devices on the exterior of the drill string is that particularly with directional drilling the exterior devices are damaged by striking the formations about the bore hole.
In order to enhance communication with the earth's surface, it is preferred to electrically isolate drill string components so that electromagnetic signals can be developed for data telemetry. This is achieved by a subassembly connector which electrically isolates adjacent drill string components so that the isolated components provide the two terminals of an antenna to which an alternating current is applied in developing the electromagnetic signal for transmission to the earth's surface. Examples of such connectors are described in U.S. Pat. No. 6,050,353 to Ryan Energy, U.S. Pat. No. 5,138,313 to Haliburton Company, U.S. Pat. No. 5,163,714 to Geoservice and Canadian patent application 2,151,525 to McAllister Petroleum Services, Ltd.
The various types of subassemblies provide for electrical isolation which are particularly useful in bore hole inspection, but may be subject to failure when used, for example, in directional drilling. It has been found that the drill string, and in particular the subassembly connector, is subjected to extreme torsional compression, tension, and bending moments during directional drilling. Such extreme forces can result in connector failure, usually at the weakest point in the subassembly. The connectors of these patents and patent application may fail due to overstressing and possibly break up at their weakest point. Furthermore, in the prior art, such as U.S. Pat. No. 4,766,442, it is generally accepted that to prevent short circuiting of the alternating current applied to the subassembly, a substantial gap spacing (i.e. 50 cm or more) is necessary. These large gaps require a protective wrapping, as the abrasive conditions during drilling can quickly damage the insulative materials used in these gaps. However, the protective wrapping is also subject to extreme abrasive forces and are consequently prone to frequent failure, thus necessitating frequent replacement.